In my experience, constructive criticism can be accepted as such far more readily when it's sweetened with some credit where it is due as well as highlighting the negatives.
Things that are useless, from my POV:
"This is great!"
"This is shit! DIAF! Also, your momma."
"I hated this / enjoyed this."
"This is awesome and everyone who disagrees is an idiot."
OK, so unmodified praise is nice, admittedly, but beyond stroking your ego a little (and fuck, it so needs it a lot of the time!) it doesn't help you improve things one iota. 99% of all opinion on the Internet falls into the utterly useless category one way or another; whether it's unmitigated fanboyism or fangirlism; or the usual mouth-foaming rabid hatred and ignorance that accompanies anything published anywhere publicly where comments are allowed.
People seem to think that they have this colossal right to not only have opinions, but express them too. Something about democracy and freedom of speech. Go figure. This mostly seems to equate to the notion that you have the inalienable right to be a colossal cunt, as evidenced by the most cursory of glances at youTube comments.
In my ideal world, though, people would offer up criticism and actually say why they think things have worked or haven't worked. They won't reach straight for the rhetoric and the hyperbole. Instead they will say things like:
"I enjoyed this because you did x really well. Y wasn't a bad idea, but was less effective because <insert reasoning>."
"I really didn't like this at all because <insert reasoning>. Still, z was quite effective when you did <something>."
Everyone has opinions, but qualified opinions are gold.